Influenza virus infection is a major public health problem, causing millions of cases of severe illness and as many as 500,000 deaths each year worldwide (WHO report, 2004, A56/23). Influenza virus has A, B and C types, among which the type A can be further classified into many sub-types according to the variations in NA and HA genes. Thus far, there have been 15 HA subtypes and 9 NA subtypes and the different combinations between HA and NA subtypes can form many types of influenza A virus subtypes.
Although inactivated vaccines are 60-80% effective against the matched influenza strains, vaccination coverage is a problem worldwide. Moreover, this strategy provides no protection against unexpected strains, outbreaks such as the H5 and H7 avian influenza outbreaks in Hong Kong in 1997 and the Netherlands and Southeast Asia in 2003-2004, or pandemics. Currently, antiviral drugs are the best defense against these outbreaks, but they provide only partial protection (Nicholson, etc., Lancet, 355:1845-1850, 2000), usually companied with some side effects, especially to the central nervous system (Wenzel, JAMA, 283:1057-1059, 2000).